The present invention relates to length measuring systems for measuring or adjusting the relative position of two objects. In particular, this invention is suitable for use with encapsulated length measuring systems which incorporate a measuring scale arranged on a carrier, in which the measuring scale includes a graduation mounted on a graduation carrier, and in which the graduation carrier is movable slightly in the measuring direction substantially independently of the carrier in the event of thermally induced changes in length. Typically, such carriers are connected by means of fastening elements with one of the two objects, the position of which is to be measured. The graduation carrier of the scale may consist for example of a rigid glass bar which is mounted to the carrier by means of an elastic adhesive layer. Alternately, the graduation carrier may be formed of a flexible metal band which is mounted in a groove defined by the carrier so as to be held in position in a manner substantially free of distorting forces.
From West German DE-AS No. 26 43 304 it is known to provide an encapsulated length measuring system for measuring the relative position of two objects in which a measuring scale which defines a graduation rests on the edges of a longitudinal channel defined in the interior of a hollow profile housing. In the disclosed system the measuring scale is fastened in its middle by means of an adhesive of very low elasticity to the edges of the longitudinal channel, and in other places by means of an adhesive of high elasticity. The hollow profile is mounted at each end to an object, in each case by means of a fastening element which defines one or more weakened places in the form of grooves which are provided to avoid undesirable tension in the fastening of the hollow profile to the attachment surface of the object to be measured. In this measuring arrangement the measuring scale, which is made up of the graduation carrier and the graduation, defines at its middle section a fixed or zero point with respect to the hollow profile. In the event of thermally induced changes in length of the hollow profile with respect to the object to be measured, longitudinal forces extending along the measuring direction may be induced in the hollow profile because of the rigid fastening of the hollow profile at both ends to the object to be measured. Such induced longitudinal forces may lead to a displacement of the fixed zero point and thereby to inaccuracies in measuring.
West German DE-OS No. 28 53 771 discloses a length measuring system which includes a housing which is mounted at both ends of a fastening element to an object to be measured. In this system, one end of the housing is joined directly by means of a first fastening element to the object to be measured, and a length compensating element is provided between the other end of the housing and a second fastening element. This length compensating element operates in such a manner that the associated end of the housing is mounted to the object to be measured with a translatory degree of freedom extending along the longitudinal direction of the housing. The disclosed system includes a measuring scale mounted in the interior of the housing. In order to avoid zero point displacements of the scale with respect to the housing in the event of temperature changes, the scale should be solidly joined with the housing at the same end of the housing as that at which the housing is directly mounted to the object to be measured.
If, on the other hand, the measuring scale is mounted to the housing over its entire length, for example with an elastic adhesive layer, and not fixed at one end (so that the scale in the event of thermally induced length variations is movable substantially independently of the housing in the measuring direction) a fixed or zero point with respect to the housing is provided near the center of the measuring scale. Because one end of the housing is directly fastened to the object to be measured by the first fastening element, thermally induced length variations in the housing result in a greater or lesser displacement of all points of the scale with respect to the object to be measured, in the event the housing and the object to be measured have differing coefficients of thermal expansion.
The document of the firm of Dr. Johannes Heidenhain GmbH "Messtechnische Information", 8th edition, March, 1980, in the article "Path Measuring Systems for Large Machine Tools" by Alfons Ernst, describes at page 33 length measuring systems which, as shown in FIGS. 1-3, include housings which are joined at both ends by means of fastening elements with the object to be measured. In the case of large measuring lengths, further fastening angle pieces are provided for the casing between the fastening elements at the ends of the casing. These further fastening angle pieces are provided to avoid any tendency of the housing to sag as a consequence of its own weight.